A concentration detector using a quartz resonator is known as a method of sensing a very small amount of substance. The concentration detector makes up a quartz sensor (concentration sensor) by forming an adsorbing layer to adsorb a sensing target on the surface of the quartz resonator, so that the concentration of the sensing target is intended to determine by utilizing the property that when the sensing target is adhered to the adsorbing layer, the natural frequency thereof is varied according to the adhesion amount. More specifically, the main part is structured such that it includes an oscillation circuit connected to the quartz sensor and the measurement unit to determine the oscillation frequency of the oscillation circuit. Use of such a method has the merit that it is possible to measure even a substance in an extremely small amount because of its wide range of applicability, simplicity in device structure, and high sensitivity.
For instance, Patent Document 1 describes that the use of the quartz sensor during analyzing of a disease marker substance contained in blood, urine, and so on is an effective method substituting an immuno-latex kit which requires an expensive large automatic analyzer. When the quartz sensor is used as a biosensor in this manner, the adsorbing layer composed of an antibody which induces an antibody-antigen reaction against a sensing target substance is formed in a quartz resonator.
When the quartz sensor is immersed in a liquid, the equivalent series resistance value of the quartz piece increases to the level of, for instance, several hundred Ω, and if the liquid is pure water, it shows an increase of about 150Ω. Therefore, it is designed to give a large amount of energy to the quartz piece. However, the interelectrode capacitance C0 of the quartz resonator consumes this energy, which results in energy loss. Accordingly, cancellation of the interelectrode capacitance C0 is necessary on the oscillation circuit side. However, even the interelectrode capacitance C0 of a certain quartz sensor is cancelled by an inductor on the oscillation circuit side, when the quartz sensor is changed to another quartz sensor having a different interelectrode capacitance C0, it is necessary to adjust the inductance value of the inductor on the oscillation circuit side every time. This operation requires complicated and time-consuming work such that the interelectrode capacitance C0 is measured for every type of quartz sensors with an LCR meter or the like on the user side to exchange with an inductor having a proper inductance value after having calculated based on the measurement value. Although it is also conceivable to prepare an oscillation circuit corresponding to the type of the quartz sensor to be used instead of exchanging the inductor, it requires an expensive system.
Patent Document 2 describes necessity of canceling the interelectrode capacitance by connecting inductance to a quartz resonator in parallel, but the invention is in a field of a completely different technology, and is not intended to solve the problem of the present invention.
Patent Document 1
Japanese Patent Application Laid-open No. 2001-83154: Column 0002, and 0004.
Patent Document 2
Japanese Utility Model Laid-open No. Sho 61-85920: FIG. 1